Electrolysis. Terms used in electrolysis Electrolysis is the decomposition of an electrolyte in molten state or aqueous solution by electricity. An electrolyte.

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Presentation on theme: "Electrolysis. Terms used in electrolysis Electrolysis is the decomposition of an electrolyte in molten state or aqueous solution by electricity. An electrolyte."— Presentation transcript:

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Electrolysis

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Terms used in electrolysis Electrolysis is the decomposition of an electrolyte in molten state or aqueous solution by electricity. An electrolyte is a substance which conducts an electric current in molten state or aqueous solution, and is decomposed by electricity. The anode is the electrode where oxidation occurs. It is the electrode connected to the positive terminal of the d.c. supply. The cathode is the electrode where reduction occurs. It is the electrode connected to the negative terminal of the d.c. supply.

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Terms used in electrolysis An anion is a negative ion and is attracted to the anode. A cation is a positive ion and is attracted to the cathode. An ammeter is an instrument used to measure the electric current passing through a circuit. Electric current is measured in ampere (A). A variable resistor (or rheostat) is used to vary the resistance and then regulate the current.

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Electrolysis

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Factors affecting electrolysis The position of ions in the electrochemical series. The concentration of ions in the solution. The nature of the electrodes.

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Case 2: Electrolysis of acidified water using platinum electrodes Dilute acid is added to provide more mobile ions so as to increase the conductivity of the water. The concentration of dilute acid increases at the end as water is consumed in the electrolysis.

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Case 3: Electrolysis of dilute sodium chloride solution using carbon electrodes The sodium ions and hydrogen ions move towards the cathode. At the cathode, the position of hydrogen ions in the electrochemical series is lower than that of sodium ions. Hydrogen ions are preferentially discharged (reduced) to form colourless hydrogen gas. 2H + (aq) + 2e –  H 2 (g)

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Case 3: Electrolysis of dilute sodium chloride solution using carbon electrodes Overall reaction: 2H 2 O( )  O 2 (g) + 2H 2 (g) Water ionizes continuously to replace the hydrogen ions discharged at the cathode. Thus there is an excess of hydroxide ions near the cathode and the solution there becomes alkaline. Water ionizes continuously to replace the hydroxide ions discharged at the anode. Thus there is an excess of hydrogen ions near the anode. The solution there becomes acidic.

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Case 3: Electrolysis of dilute sodium chloride solution using carbon electrodes If a few drops of universal indicator are added to the sodium chloride solution, the solution near the cathode will turn blue while that near the anode will turn red. The sodium chloride becomes more concentrated as water is consumed in the electrolysis.

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Case 4: Electrolysis of dilute copper(II) sulphate solution using carbon electrodes The copper(II) ions and hydrogen ions move towards the cathode. At the cathode, the position of copper(II) ions in the electrochemical series is lower than that of hydrogen ions. Copper(II) ions are preferentially discharged (reduced) to form brown copper metal. Cu 2+ (aq) + 2e –  Cu(s)

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Case 4: Electrolysis of dilute copper(II) sulphate solution using carbon electrodes The blue colour of the solution fades out because the concentration of copper(II) ions decreases. Copper(II) ions and hydroxide ions are consumed in the electrolysis. Hydrogen ions and sulphate ions remain in the solution. Thus the solution eventually becomes sulphuric acid.

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Case 4: Electrolysis of dilute copper(II) sulphate solution using carbon electrodes After a few minutes, cathode is coated with copper. If the polarities of cells are then reversed, anode is coated with copper. The factor of electrode should be considered as in case 8.

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Case 5: Electrolysis of dilute sodium iodide solution using carbon electrodes The sodium ions and hydrogen ions move towards the cathode. At the cathode, the position of hydrogen ions in the electrochemical series is lower than that of sodium ions. Hydrogen ions are preferentially discharged (reduced) to form colourless hydrogen gas. 2H + (aq) + 2e –  H 2 (g)

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Case 5: Electrolysis of dilute sodium iodide solution using carbon electrodes The iodide ions and hydroxide ions move towards the anode. At the anode: the position of hydroxide ions in the electrochemical series is higher than that of chloride ions. However, the concentration of iodide ions is much greater than that of hydroxide ions. Iodide ions are preferentially discharged (oxidized) to form iodine. 2I - (aq)  I 2 (aq) + 2e –

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Case 6: Electrolysis of conc. sodium chloride solution using carbon electrodes The sodium ions and hydrogen ions move towards the cathode. At the cathode, the position of hydrogen ions in the electrochemical series is lower than that of sodium ions. Hydrogen ions are preferentially discharged (reduced) to form colourless hydrogen gas. 2H + (aq) + 2e –  H 2 (g)

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Case 6: Electrolysis of conc. sodium chloride solution using carbon electrodes The chloride ions and hydroxide ions move towards the anode. At the anode: the position of hydroxide ions in the electrochemical series is higher than that of chloride ions. However, the concentration of chloride ions is much greater than that of hydroxide ions. Chloride ions are preferentially discharged (oxidized) to form chlorine gas. 2Cl - (aq)  Cl 2 (g) + 2e –

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Case 6: Electrolysis of conc. sodium chloride solution using carbon electrodes Overall reaction: 2H + (aq) + 2Cl – (aq)  H 2 (g) + Cl 2 (aq) Water ionizes continuously to replace the hydrogen ions discharged at the cathode. Thus there is an excess of hydroxide ions near the cathode. The solution there becomes alkaline. The chlorine gas formed at the anode dissolves in the solution. The solution there becomes acidic and has a bleaching effect.

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Case 7: Electrolysis of conc. sodium chloride solution using mercury electrodes The sodium ions and hydrogen ions move towards the cathode. At the cathode, the position of hydrogen ions in the electrochemical series is lower than that of sodium ions. However, sodium ions are preferentially discharged (reduced) to form sodium metal. The sodium metal formed dissolves in the mercury to form a sodium amalgam. Na + (aq) + e – + Hg(l)  Na/Hg(l) sodium amalgam

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Case 7: Electrolysis of conc. sodium chloride solution using mercury electrodes The chloride ions and hydroxide ions move towards the anode. At the anode: the position of hydroxide ions in the electrochemical series is higher than that of chloride ions. However, the concentration of chloride ions is much greater than that of hydroxide ions. Chloride ions are preferentially discharged (oxidized) to form chlorine gas. 2Cl - (aq)  Cl 2 (g) + 2e –

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Case 8: Electrolysis of dilute copper(II) sulphate solution using copper electrodes The copper(II) ions and hydrogen ions move towards the cathode. At the cathode, the position of copper(II) ions in the electrochemical series is lower than that of hydrogen ions. Copper(II) ions are preferentially discharged (reduced) to form brown copper metal. Cu 2+ (aq) + 2e –  Cu(s)

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Case 8: Electrolysis of dilute copper(II) sulphate solution using copper electrodes The sulphate ions and hydroxide ions move towards the anode. At the anode: the position of hydroxide ions in the electrochemical series is higher than that of sulphate ions. However, copper is a stronger reducing agent than hydroxide ions and thus more easily oxidized. The copper anode dissolves to form copper(II) ions (oxidized). Cu(s)  Cu 2+ (aq) + 2e –

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Case 8: Electrolysis of dilute copper(II) sulphate solution using copper electrodes Overall reaction: Cu(s)  Cu(s) anode cathode The net effect is the transfer of copper from the anode to the cathode. The rate at which copper deposits on the cathode is equal to the rate at which the copper anode dissolves. Increase in mass of cathode = decrease in mass of anode

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Case 8: Electrolysis of dilute copper(II) sulphate solution using copper electrodes The concentration of copper(II) ions in the solution remains the same. The blue colour of the solution does not change.

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Comparing a chemical cell and an electrolytic cell

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Chemical cellElectrolytic cell FunctionA device for generating electricity from chemical reactions. A device for bringing out chemical changes by electricity. Direction of electricity Electrons flow from negative electrode to the positive electrode through the external circuit. Circuit is completed by the movement of mobile electrons. Cations discharge and gain electrons at cathode, while anions discharge and give up electrons at anode. Circuit is completed by the movement of mobile ions.

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Manufacture of bleaching solution This process also produces waste which contains poisonous mercury compounds. These waste products will cause serious pollution problems if they are discharged into rivers and seas.

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Refining of copper Impurities such as silver, gold and platinum settle at the bottom of the container. At the cathode, the position of copper(II) ions in the electrochemical series is lower than that of hydrogen ions. Copper(II) ions are preferentially discharged (reduced) to form brown copper metal. Cu 2+ (aq) + 2e –  Cu(s)

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Solutions Controlling the pH value of effluents The pH value of acidic effluents can be controlled by adding sodium carbonate. The pH value of alkaline effluents can be controlled by adding sulphuric acid. Treatment of heavy metal compounds Add sodium hydroxide solution to the effluents to form insoluble metal hydroxides. The solid is then filtered off.

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Solutions Treatment of poisonous chromium waste Poisonous chromium(VI) compounds are reduced to non-toxic chromium(III) compounds by sodium sulphite. Sodium hydroxide solution is then added to the chromium(III) compounds to form solid chromium(III) hydroxide. The solid is then filtered off.

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Extraction of reactive metals Reactive metals such as K, Na, Ca, Mg and Al are extracted from its ores by electrolysis of molten metal ores. Metal ions are attracted to the cathode and reduced to form metal. M n+ (l) + ne -  M(s)